Question

In my Haskell program i have an ADT with many constructors:

data MyData = Con1 |
    Con2 |
    ...
    Con20

I have an foreign export ccall function, which wraps [MyData] into array of StablePtr's. After calling it i need to determine which constructor have been used to construct each element.

It could be solved this way

foreign export ccall getType :: StablePtr MyData -> IO CInt
getType (Con1) = return 1
getType (Con2) = return 2
...

but then i would need to manually define these constants in C header. This is error-prone, so i wonder if there is a way to make GHC do this job for me.

Answer 1

I’ve found solution for this.

I’ve defined possible constructor types as enum in my C code:

typedef enum
{
    MyDataCon1,
    MyDataCon2,
    ...
    MyDataCon20
} MyDataConstructor;

Then i used C->Haskell enum hook in my Haskell source:

{#enum MyDataConstructor deriving (Show) #}

After preprocessing this line turns to

data MyDataConstructor = MyDataCon1
    | MyDataCon2
    ...
    | MyDataCon20

Now i can define getType this way:

foreign export ccall getType :: StablePtr MyData -> IO CInt
getType md = do
    md' <- deRefStablePtr md
    case md' of
        Con1 -> return $ fromEnum MyDataCon1
        Con2 -> return $ fromEnum MyDataCon2
        ...
        Con20 -> return $ fromEnum MyDataCon20